1. Field of the Invention
The present invention relates to a golf ball achieving both excellent hit feeling and good restitution performance.
2. Description of the Background Art
A core of a golf ball has conventionally been manufactured by heating a rubber composition, obtained by mixing unsaturated carboxylic acid metal salt serving as a co-crosslinking agent and a free radical initiator with a rubber component mainly consisting of polybutadiene, so as to form crosslinking among main chains of rubber molecules. As the unsaturated carboxylic acid metal salt, zinc acrylate or zinc methacrylate has generally been used. Co-crosslinking is formed by graft polymerization of the unsaturated carboxylic acid metal salt and a main chain of polybutadiene by a free radical initiator such as dicumyl peroxide. Here, a dispersion state of the unsaturated carboxylic acid metal salt in the rubber composition and a reaction speed of crosslinking to the main chain of the rubber molecule considerably affect a fundamental physical property of the rubber composition after crosslinking as well as characteristics of a golf ball fabricated with the same.
Therefore, improvement in dispersibility of zinc acrylate in the rubber composition by coating a particle surface of the zinc acrylate with a higher fatty acid or higher fatty acid metal salt has conventionally been proposed (see U.S. Pat. No. 4,561,657 and Japanese Patent Laying-Open No. 60-92781).
In addition, as a method of improving dispersibility of the unsaturated carboxylic acid metal salt in the rubber composition by using the unsaturated carboxylic acid metal salt of which mean particle size is controlled, two methods have been proposed: a method of using the unsaturated carboxylic acid metal salt having a mean particle size of not larger than 51 μm (see Japanese Patent Laying-Open No. 8-196661 and Japanese Patent Laying-Open No. 11-57068); and a method of using as a co-crosslinking agent, unsaturated carboxylate having size distribution of 0.1 to 5 μm and a mean particle size of 1 to 4.5 μm (see Japanese Patent Laying-Open No. 9-235413 and U.S. Pat. No. 6,136,906).
These methods are preferable in order to improve dispersibility of the co-crosslinking agent in the rubber composition and increasing hardness of the rubber composition. With these methods, however, the co-crosslinking agent is finely dispersed, and accordingly, crosslinking density among the main chains of the rubber molecules giving the largest contribution to the restitution performance is lowered. On the other hand, graft polymerization pattern of the co-crosslinking agent and the main chains of the rubber molecules that do not give much contribution to the restitution performance is increased, whereby the restitution performance is not sufficiently achieved.
Meanwhile, another method of improving the restitution performance of the rubber composition has been proposed (see U.S. Publication No. 2002/0086742A1), in which α,β-unsaturated carboxylic acid metal salt having a carbon number of 3 to 8 and the mean particle size of 6 to 30 μm is used as the co-crosslinking agent so as to avoid excessively large surface area, and the co-crosslinking agent is blended by an amount of 15 to 50 parts by weight with respect to a base rubber of 100 parts by weight so as to adjust reactivity of the co-crosslinking agent with the base material. With this method, however, radicals are produced by an initiator in the presence of the co-crosslinking agent. Therefore, crosslinking among polymers, reaction of a polymer with the co-crosslinking agent and reaction among co-crosslinking agents simultaneously occur. In addition, heat is generated by polymerization and fluidity of the polymer is increased, whereby the co-crosslinking agent that initially aggregated will be dispersed. Moreover, since there is a temperature difference between the inside and the outside of the core, it is difficult to maintain the state of the co-crosslinking agent constant. From the above-described reasons, it is difficult to achieve sufficient restitution performance.